This invention relates generally to optical communication systems. More specifically, it relates to a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for wavelength division multiplexed optical networking applications.
As fiber-optic communication networks rapidly spread into every walk of modern life, there is a growing demand for optical components and subsystems that enable the fiber-optic communications networks to be increasingly scalable, versatile, robust, and cost-effective.
Contemporary fiber-optic communications networks commonly employ wavelength division multiplexing (WDM), for it allows multiple information (or data) channels to be simultaneously transmitted on a single optical fiber by using different wavelengths and thereby significantly enhances the information bandwidth of the fiber. The prevalence of WDM technology has made optical add-drop multiplexers indispensable building blocks of modern fiber-optic communication networks. An optical add-drop multiplexer (OADM) serves to selectively remove (or drop) one or more wavelengths from a multiplicity of wavelengths on an optical fiber, hence taking away one or more data channels from the traffic stream on the fiber. It further adds one or more wavelengths back onto the fiber, thereby inserting new data channels in the same stream of traffic. As such, an OADM makes it possible to launch and retrieve multiple data channels (each characterized by a distinct wavelength) onto and from an optical fiber respectively, without disrupting the overall traffic flow along the fiber. Indeed, careful placement of the OADMs can dramatically improve an optical communication network""s flexibility and robustness, while providing significant cost advantages.
Conventional OADMs in the art typically employ multiplexers/demultiplexers (e.g, waveguide grating routers or arrayed-waveguide gratings), tunable filters, optical switches, and optical circulators in a parallel or serial architecture to accomplish the add and drop functions. In the parallel architecture, as exemplified in U.S. Pat. No. 5,974,207, a demultiplexer (e.g., a waveguide grating router) first separates a multi-wavelength signal into its constituent spectral components. A wavelength switching/routing means (e.g., a combination of optical switches and optical circulators) then serves to drop selective wavelengths and add others. Finally, a multiplexer combines the remaining (i.e., the pass-through) wavelengths into an output multi-wavelength optical signal. In the serial architecture, as exemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g., Bragg fiber gratings) in combination with optical circulators are used to separate the drop wavelengths from the pass-through wavelengths and subsequently launch the add channels into the pass-through path. And if multiple wavelengths are to be added and dropped, additional multiplexers and demultiplexers are required to demultiplex the drop wavelengths and multiplex the add wavelengths, respectively. Irrespective of the underlying architecture, the OADMs currently in the art are characteristically high in cost, and prone to significant optical loss accumulation. Moreover, the designs of these OADMs are such that it is inherently difficult to reconfigure them in a dynamic fashion.
U.S. Pat. No. 6,204,946 to Askyuk et al. discloses an OADM that makes use of free-space optics in a parallel construction. In this case, a multi-wavelength optical signal emerging from an input port is incident onto a ruled diffraction grating. The constituent spectral channels thus separated are then focused by a focusing lens onto a linear array of binary micromachined mirrors. Each micromirror is configured to operate between two discrete states, such that it either retroreflects its corresponding spectral channel back into the input port as a pass-through channel, or directs its spectral channel to an output port as a drop channel. As such, the pass-through signal (i.e., the combined pass-through channels) shares the same input port as the input signal. An optical circulator is therefore coupled to the input port, to provide necessary routing of these two signals. Likewise, the drop channels share the output port with the add channels. An additional optical circulator is thereby coupled to the output port, from which the drop channels exit and the add channels are introduced into the output port. The add channels are subsequently combined with the pass-through signal by way of the diffraction grating and the binary micromirrors.
Although the aforementioned OADM disclosed by Askyuk et al. has the advantage of performing wavelength separating and routing in free space and thereby incurring less optical loss, it suffers a number of limitations. First, it requires that the pass-through signal share the same port/fiber as the input signal. An optical circulator therefore has to be implemented, to provide necessary routing of these two signals. Likewise, all the add and drop channels enter and leave the OADM through the same output port, hence the need for another optical circulator. Moreover, additional means must be provided to multiplex the add channels before entering the system and to demultiplex the drop channels after exiting the system. This additional multiplexing/demultiplexing requirement adds more cost and complexity that can restrict the versatility of the OADM thus-constructed. Second, the optical circulators implemented in this OADM for various routing purposes introduce additional optical losses, which can accumulate to a substantial amount. Third, the constituent optical components must be in a precise alignment, in order for the system to achieve its intended purpose. There are, however, no provisions provided for maintaining the requisite alignment; and no mechanisms implemented for overcoming degradation in the alignment owing to environmental effects such as thermal and mechanical disturbances over the course of operation.
U.S. Pat. No. 5,906,133 to Tomlinson discloses an OADM that makes use of a design similar to that of Aksyuk et al. There are input, output, drop and add ports implemented in this case. By positioning the four ports in a specific arrangement, each micromirror (being switchable between two discrete positions) either reflects its corresponding channel (coming from the input port) to the output port, or concomitantly reflects its channel to the drop port and an incident add channel to the output port. As such, this OADM is able to perform both the add and drop functions without involving additional optical components (such as optical circulators used in the system of Aksyuk et al.). However, because a single drop port is designated for all the drop channels and a single add port is designated for all the add channels, the add channels would have to be multiplexed before entering the add port and the drop channels likewise need to be demutiplexed upon exiting from the drop port. Moreover, as in the case of Askyuk et al., there are no provisions provided for maintaining requisite optical alignment in the system, and no mechanisms implemented for combating degradation in the alignment due to environmental effects over the course of operation.
As such, the prevailing drawbacks suffered by the OADMs currently in the art are summarized as follows:
1) The wavelength routing is intrinsically static, rendering it difficult to dynamically reconfigure these OADMs.
2) Add and/or drop channels often need to be multiplexed and/or demultiplexed, thereby imposing additional complexity and cost.
3) Stringent fabrication tolerance and painstaking optical alignment are required.
Moreover, the optical alignment is not actively maintained, rendering it susceptible to environmental effects such as thermal and mechanical disturbances over the course of operation.
4) In an optical communication network, OADMs are typically in a ring or cascaded configuration. In order to mitigate the interference amongst OADMs, which often adversely affects the overall performance of the network, it is essential that the optical power levels of spectral channels entering and exiting each OADM be managed in a systematic way, for instance, by introducing power (or gain) equalization at each stage. Such a power equalization capability is also needed for compensating for non-uniform gain caused by optical amplifiers (e.g., erbium doped fiber amplifiers) in the network. There lacks, however, a systematic and dynamic management of the optical power levels of various spectral channels in these OADMs.
5) The inherent high cost and optical loss further impede the wide application of these OADMs.
In view of the foregoing, there is an urgent need in the art for optical add-drop multiplexers that overcome the aforementioned shortcomings in a simple, effective, and economical construction.
The invention provides a polarization diversity wavelength-separating-routing (WSR) apparatus and method which minimizes insertion loss and polarization-dependent loss (PDL).
In WSR apparatus with which the invention may be used, a multi-wavelength optical signal is provided from an input port to a wavelength-separator which separates the multi-wavelength optical signal by wavelength into multiple spectral channels. Each channel may be characterized by a distinct center wavelength and associated bandwidth. A beam-focuser may focus the spectral channels into corresponding spots onto a plurality of channel micromirrors positioned such that each channel micromirror receives one of the spectral channels. The channel micromirrors are individually controllable and movable, e.g., continuously pivotable or rotatable, so as to reflect the spectral channels into selected ones of the output ports. Each output port may receive any number of the reflected spectral channels.
In one aspect, the WSR apparatus of the invention employs a polarization diversity arrangement to overcome polarization-sensitive effects the constituent optical elements may possess. A polarization-displacing unit and a polarization-rotating unit may be disposed along the optical path between the fiber collimators providing the input and output ports and the wavelength-separator which separates the input multi-wavelength optical signal into the constituent wavelengths. The polarization-displacing unit decomposes the input multi-wavelength optical signal into first and second polarization components. The polarization-rotating unit may subsequently rotate the polarization of the second polarization component so that its polarization is substantially parallel to the first polarization component, e.g., by 90-degrees. The wavelength-separator separates the incident optical signals by wavelength into first and second sets of optical beams, respectively. The beam-focuser may focus the first and second sets of optical beams into corresponding focused spots, impinging onto the channel micromirrors. The first and second optical beams associated with the same wavelength may impinge onto (and be manipulated by) the same channel micromirror. The channel micromirrors may be individually controlled such that the first and second sets of optical beams are deflected, upon reflection. The reflected first set of optical beams may subsequently undergo a rotation in polarization by, e.g., 90 degrees, by the polarization-rotating unit. This enables the polarization-displacing unit to recombine the reflected first and second sets of optical beams by wavelength respectively into reflected spectral channels, prior to being coupled into the output ports.
The polarization-displacing unit may comprise one or more polarization-displacing elements, each being a birefringent beam displacer, or a polarizing-beam-splitting element, e.g., a polarizing beam splitter in conjunction with a suitable beam-reflector. The polarization-rotating unit may include one or more polarization rotating elements, each being a half-wave plate, a Faraday rotator, or a liquid crystal rotator known in the art.
A distinct feature of the channel micromirrors in the WSR apparatus is that the motion of each channel micromirror is under analog control such that its pivoting angle can be continuously adjusted. This enables each channel micromirror to scan its corresponding spectral channel across all possible output ports and thereby direct the spectral channel to any desired output port.
In the WSR apparatus, the wavelength-separator may be a ruled diffraction grating, a holographic diffraction grating, an echelle grating, a curved diffraction grating, a transmission grating, a dispersing prism, or other wavelength-separating means known in the art. The beam-focuser may be a single lens, an assembly of lenses, or other beam-focusing means known in the art. The channel micromirrors may be silicon micromachined mirrors, reflective ribbons (or membranes), or other types of beam-deflecting means known in the art. Each channel micromirror may be pivotable about one or two axes. Fiber collimators serving as the input and output ports may be arranged in a one-dimensional or two-dimensional array. In the latter case, the channel micromirrors may be pivotable biaxially.
In another aspect, the WSR apparatus of the invention may comprise an array of collimator-alignment mirrors, in optical communication with the wavelength-separator and the fiber collimators, for adjusting the alignment of the input multi-wavelength signal and for directing the spectral channels into the selected output ports by way of angular control of the collimated beams. Each collimator-alignment mirror may be rotatable about one or two axes. The collimator-alignment mirrors may be arranged in a one-dimensional or two-dimensional array. First and second arrays of imaging lenses may additionally be optically interposed between the collimator-alignment mirrors and the fiber collimators such that the collimator-alignment mirrors are effectively xe2x80x9cimagedxe2x80x9d onto the corresponding fiber collimators to ensure an optimal alignment.
In another aspect, the WSR apparatus of the invention may include a servo-control assembly, in communication with the channel micromirrors and the output ports. The servo-control assembly serves to monitor the optical power levels of the spectral channels coupled into the output ports and further provide control of the channel micromirrors on an individual basis, so as to maintain a predetermined coupling efficiency of each spectral channel into one of the output ports. As such, the servo-control assembly provides dynamic control of the coupling of the spectral channels into the respective output ports and actively manages the optical power levels of the spectral channels coupled into the output ports. (If the WSR apparatus includes an array of collimator-alignment mirrors as described above, the servo-control assembly may additionally provide dynamic control of the collimator-alignment mirrors.) Moreover, the utilization of such a servo-control assembly effectively relaxes the requisite fabrication tolerances and the precision of optical alignment during assembly of a SR apparatus of the invention, and further enables the system to correct for shift in optical alignment over the course of operation. A WSR apparatus incorporating a servo-control assembly thus described is termed a WSR-S apparatus, in the following discussion.
The WSR apparatus of the invention affords a variety of optical devices, including a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs), that provide many advantages over the prior art devices, notably:
1) By advantageously employing an array of channel micromirrors that are individually and continuously controllable, an OADM of the invention is capable of routing the spectral channels on a channel-by-channel basis and directing any spectral channel into any one of the output ports. As such, its underlying operation is dynamically reconfigurable, and its underlying architecture is intrinsically scalable to a large number of channel counts.
2) The add and drop spectral channels need not be multiplexed and demultiplexed before entering and after leaving the OADM respectively. And there are not fundamental restrictions on the wavelengths to be added or dropped.
3) The coupling of the spectral channels into the output ports is dynamically controlled by a servo-control assembly, rendering the OADM less susceptible to environmental effects (such as thermal and mechanical disturbances) and therefore more robust in performance. By maintaining an optimal optical alignment, the optical losses incurred by the spectral channels are also significantly reduced.
4) The optical power levels of the spectral channels coupled into the output ports can be dynamically managed according to demand, or maintained at desired values (e.g., equalized at a predetermined value) by way of the servo-control assembly. This spectral power-management capability as an integral part of the OADM will be particularly desirable in WDM optical networking applications.
5) The use of free-space optics provides a simple, low loss, and cost-effective construction. Moreover, the utilization of the servo-control assembly effectively relaxes the requisite fabrication tolerances and the precision of optical alignment during initial assembly, enabling the OADM to be simpler and more adaptable in structure, and lower in cost and optical loss.
6) The use of a polarization diversity scheme renders the polarization-sensitive effects inconsequential in the OADM. This enables the OADM to minimize the insertion loss; and enhance spectral resolution in a simple and cost-effective construction (e.g., by making use of high-dispersion diffraction grating commonly available in the art). The polarization diversity scheme further allows the overall optical paths of the two polarization components for each spectral channel to be substantially equalized, thereby minimizing the polarization-dependent loss. Such attributes would be particularly desirable in WDM optical networking applications.